The present invention relates to bicycle derailleur shifting systems and more particularly to a chainwheel assembly including a larger diameter chainwheel having at least one tooth group configured to reduce noise when shifting a chain from a smaller chainwheel to the larger chainwheel.
A conventional derailleur shifting system includes a driving chainwheel assembly, a driven chainwheel assembly and a chain extending therebetween. At least one of the chainwheel assemblies includes a plurality of chainwheels having varying tooth counts. The chain includes successive pins connecting pairs of inner and outer link plates arranged in alternating sequence. A derailleur shifts the chain between chainwheels. As the chain is shifted from a smaller chainwheel to a larger chainwheel, it forms a transition region extending from a chain link that is last to engage between the two teeth of the smaller diameter chainwheel to a chain link that is first to engage between two teeth of the larger chainwheel. The first tooth on the larger chainwheel, in the drive rotation direction, before the space into which a chain link would fit during the shifting operation is referred to as the reference tooth, while the tooth that is the first to penetrate between a pair of link plates is referred to as the chain capture tooth. During the shifting of the chain from a smaller chainwheel to a larger chainwheel, the chain capture tooth first fits between a pair of outer link plates because the spacing between the outer link plates is wider than the spacing between a pair of inner link plates. During the shifting operation, a pair of inner link plates presses tightly against the tooth located before the chain capture tooth, in the drive rotation direction, thus facilitating penetration of the chain capture tooth between the outer link plate pair. Since the gap between the inner link plates is narrow, a centerline of the transition region of the chain comes closer to the chain capture tooth when the inner link plate is against the larger chainwheel than when an outer link plate is against the larger chainwheel.
To facilitate the shifting of the chain from the smaller chainwheel to the larger chainwheel, features have been provided on the teeth of the larger chainwheel to define the course of the transition region of the chain. For example, U.S. Pat. No. 4,268,259 discloses a chainwheel assembly engagable by a roller chain for a derailleur shifting system. To improve the shifting operation, a smaller chainwheel and a larger chainwheel are arranged such that a center between a pair of adjacent teeth on the larger chainwheel is positioned on a tangent extending from the center between a pair of adjacent teeth on the smaller chainwheel. The distance between the two centers is equal to an integer multiple of the chain pitch. With this configuration, the first tooth that is behind the center on the larger chainwheel, in the driving rotation direction, is the chain capture tooth. When a pair of outer link plates arrives at the chain capture tooth, the chain capture tooth penetrates into the wide space between the outer link plates. However, when a pair of inner link plates arrives at the chain capture tooth, the chain capture tooth does not penetrate into the narrow space between the inner link plates. Thus the following tooth that also has a configuration of a chain capture tooth penetrates the wide space between the next pair of outer link plates. Having two adjacent chain capture teeth and the tangential spacing between the larger and smaller chainwheels facilitates the shifting of the chain from the smaller to the larger chainwheel.
European Patent No. 0 313 345 discloses another example of a chainwheel assembly engageable by a roller chain for a derailleur shifting system. To improve the shifting of the chain from a smaller chainwheel to a larger chainwheel, the two chainwheels are oriented to have defined tooth positions with respect to each other and certain teeth on the larger chainwheel have recesses on a front face of the larger chainwheel facing the smaller chainwheel. Additionally, the larger chainwheel includes at least one tooth, after the recess in the drive rotation direction, configured to fit into the gap between the outer link plates. Thus, this tooth functions as a chain capture tooth. If a pair of inner link plates is located at the chain capture tooth, the chain capture tooth cannot fit into the narrow gap between the inner link plates. The inner link plate runs laterally past the chain capture tooth and the next tooth becomes the chain capture tooth that fits into the wider gap of the outer link plates. Accordingly, three teeth may be configured as chain capture teeth or chain capture teeth, while the remaining teeth are configured to be nonengaging teeth. The depth of the recesses approximately equals a thickness of the link plate and are configured as run-in or run-on ramps that facilitate the shifting of the chain from a smaller chainwheel to a larger chainwheel.
One drawback of the above chainwheel designs is that they cause shifting noises. A shifting of the chain from a smaller chainwheel to a larger chainwheel is accomplished by the derailleur laterally engaging the chain at an unloaded or untension segment of the chain, allowing the shifting operation to be initiated with low shifting forces and the untension chain segment to move to the larger chainwheel. At the same time, the tension segment of the chain is still located on the smaller chainwheel. However, as the chainwheel assembly continues to rotate, the previously untension chain segment moves into the tension chain segment. The tension segment of the chain rotates away from the last tooth of the smaller chainwheel and shifts from the smaller chainwheel to the larger chainwheel. This changes the axial position of the chain on the chainwheel assembly. When the tension segment of the chain shifts to the larger chainwheel, the chain slides up laterally on the large chainwheel and then jumps over the last tooth located before the chain capture tooth, rubbing over a back of the tooth, resulting in noise. Shifting noises are mainly caused by the tension segment of the chain. The greater the tension or load, the greater the angle between the chain segment sliding up laterally on the larger chainwheel tooth and the chain segment running off the larger chainwheel after the shifting operation, the greater the noise.